When laying a new utility cable or pipe underground, an underground drill can be used to avoid the need to dig and fill-in a trench. When performing such trenchless drilling, the drill operator must be aware of the exact location of the drill head, in order to steer the drill in the correct direction and to avoid buried utilities or underground structures.
To locate the drill head, a sonde can be inserted therein. The sonde generates an alternating dipole magnetic field with a frequency of oscillation in the range 200 Hz to 83 KHz. At the lower of these frequencies, attenuation due to the drill bit enclosure and distortion due to ground effects are minimised. The magnetic signal can be modulated to transmit useful information, such as data relating to the tilt and roll angle of the sonde, a temperature reading and/or a battery life reading.
The magnetic field produced by the sonde is detectable by a portable detector, which can be hand-held or mounted on a tripod. The detector incorporates an array of highly sensitive antennas which detect electromagnetic signals from 50 Hz up to around 250 kHz. The signal generated in each antenna is processed to identify and isolate the sonde signal. The field magnitude, field gradient and relative phase of the sonde signal detected at each antenna are evaluated and these measurements are processed to determine the location and orientation of the sonde and associated drill head. The detector incorporates a digital signal processor (DSP) to process the received electromagnetic signals. The degree of complexity of the algorithms used in the DSP depends on a number of factors, including the power available from the detector's batteries, the requirement to provide a reasonable drilling speed and the desired degree of accuracy when pinpointing the sonde.
Prior art apparatus for sonde recognition and location, comprises an analogue or digital receiver of either super-heterodyne or homodyne architecture, a phase sensitive signal processing system and use of a complex discrete Fourier transform (CDFT) to arbitrate the sonde present criterion. Such processes were limited to measuring the signal to noise ratio (SNR) of the carrier frequency with respect to the broadband noise floor. The effects of use of Blackman or Hamming window filters to suppress the spectral leakage which is inherent to the CDFT algorithm for frequency components which are not an integral number of periods within the transform length are shown in FIG. 6 which shows a sonde spectrum processed through a fast Fourier transform (FFT) using a Hamming window. Comparison with FIG. 3, which shows a typical frequency spectrum of a magnetic signal produced by a sonde, illustrates the advantage of the window function which has pushed the overall noise floor down by ˜6 dB. Although useful, these window functions introduce distortion and non-linearity into the process which is undesirable.
It is an aim of the present invention to provide apparatus and a computer program for and a method of improving the sensitivity of a detector for a sonde, to provide more accurate and faster location of a sonde and to provide detection of a sonde at a greater separation between a sonde and a detector, compared with that achievable with currently available detectors. This enhanced sonde recognition is achievable without a significant increase in power consumption by the detector, so that new algorithm does not adversely affect the battery life of the device.
Whereas the maximum distance at which currently available sondes can be detected is around 20 m, by using a new algorithm the present invention can achieve a maximum working separation between a sonde and a detector of around 28 m, with a drill speed comparable to the drill speed achievable with currently available sondes at a 20 m separation. As the strength of the magnetic field produced by a sonde at a distance along the axis of the magnetic field is governed by an inverse cube relationship, such an increase in the maximum working separation is a substantial improvement.